Method of fracturing a formation traversed by a well



SEARQH RUUM H. J. TADEMA 3,155,161

METHOD OF FRACTURING A FORMATION TRAVERSED BY A WELL Nov. 3, 1964 FiledNOV. 1, 1960 FIG. IB

FIG. IA

FIG.

FIG. 2

INVENTORI HARCO J. TADEMA MK HIS ATTORNEY United States Patent 3,155,161METHOD OF FRACTURING A FORMATION TRAVERSED BY A WELL Harco J. Tadema,Delft, Netherlands, assignor to Shell Oil Company, New York, N.Y., acorporation of Delaware Filed Nov. 1, 1960, Ser. No. 66,488 Claimspriority, application Netherlands, Nov. 13, 1959, 245,336 1 Claim. (Cl.166-42) The invention relates to a method of fracturing a formationtraversed by a well by means of a fracturing fluid.

In order to improve the production of oil and gas from subsurfaceformation traversed by a well it has been the practice to enlarge orcreate flow channels or fractures in the formations. Fractures arecreated or enlarged by the application of high pressures to fluidsdisposed in the wellbore adjacent the portion of the formation to betreated. When the pressure of the fluid exceeds the breakdown pressureof the formation the formation will fracture. In formations which tendto fracture along a horizontal plane such as loose-grained materialhaving a low rock bonding strength the breakdown pressure equals thegeostatic pressure plus any rock-bonding strength. In formations thattend to fracture along vertical planes the breakdown pressure is 60 to70 percent of the geostatic pressure plus rock bonding strength.

Fracturing the formation in this way increases the surface area of theformation in contact with the well thereby promoting a ready exchange ofmedia between the formation and the well this being important both inthe production of liquid or gas from the formation and in variousprocesses for specific treatments of the formation.

It is often desirable to produce the fracture at a certain elevation ina formation within the well. Thus, for example, in some oil-bearingformations the fracture should be located as low as possible in theformation to minimize the simultaneous production of gas and oil.

In one method of fracturing two packer elements are set at the requiredelevation in the formation thus forming in the well an isolated zonebounded by the packer elements and the wall of the formation. Byinjecting fracturing fluid under pressure by means of tubing extendinginto this zone the formation is fractured at the elevation thereof.While this is a satisfactory method it requires the installation andremoval of two packing elements and a tubing string from the well. Thisincreases the cost and expense of the fracturing operation.

Accordingly, an object of this invention is to provide a method offracturing a formation traversed by a well at a desired place which canbe carried out by the use of simple equipment.

A further object of this invention is to provide a novel method forfracturing a formation traversed by a well at a desired location byusing fluids having different specific gravities and controlling thespecific gravity of the fliud in the well so that the hydrostaticpressure of the fluid at the desired location equals the formationbreakdown pressure.

According to the invention, the bottom of the column of fracturing fluidshould be adjacent the area at which the bottom of the fracture in theformation is desired, and moreover a pressure gradient in the fracturingfluid should be created that the hydrostatic pressure of the fracturefluid exceeds the breakdown pressure of the part of the formation to befractured. Measured in a downward direction, the pressure increase inthe column of fracturing fluid per unit of length of the well is greaterthan the increase in the breakdown pressure in the formation per unit oflength of the well. The difference be- 3,155,161 Patented Nov. 3, 1964ice tween the hydrostatic pressure of the fluid at a certain level andthe breakdown pressure of the formation at the same level thereforedecreases towards the bottom of the column of fracturing fluid. When thehydrostatic pressure is increased (for example) by the introduction offluid at the top of the column of fracturing fluid, or by exertingpressure on the top of the well) a local equilibrium between thehydrostatic pressure of the fluid and the breakdown pressure will bereached first at the bottom of the column of fracturing fluid. When thefluid pressure increases still further, the fracturing of the formationwill be initiated at the bottom of the column of fracturing fluid and,depending on the state of stress prevailing in the formation, willextend in a horizontal plane or one or more vertical planes.

It is preferable to control the pressure gradient in the column offracturing fluid by means of the specific gravity of the fracturingfluid. Also a column of driving fluid having a lower specific gravitymay be introduced above the fracturing fluid to insure that thehydrostatic pressure of the fluid will not exceed the breakdown pressureof the formation except at the desired location of the fracture.

The desired level of the lowest part of the column of fracturing fluidrelative to the formation may be adjusted or positioned by means of apacker element closing off the whole passageway of the well below thepoint at which it is desired to produce the fracture.

The properties of the fracturing fluid should also be such that whenthere is suflicient pressure a fracture can be produced in theformation. These properties, such as controllable viscosity and theability to transport propping agents, are outside the scope of theinvention.

The above and additional advantages of this invention will be moreeasily understood by those skilled in the art from the followingdetailed description of a preferred embodiment when taken in conjunctionwith the attached drawing in which:

FIGURE 1 is a diagrammatic representation of a formation traversed by awell illustrating one embodiment of this invention;

FIGURES 1A and 1B illustrate graphically the value of the breakdownpressure and the value of the hydrostatic pressure of the column offracturing fluid in the well as a function of the depth of the well;and,

FIGURE 2 is a diagrammatic representation of a formation traversed by awell showing a second embodiment of this invention.

Referring to FIGURE 1, the wall of the well 10 is lined from the top ofthe well to the top of the formation 12 to be treated with a casing 13.The top of the casing 13 is closed and provided with a connection 14 viawhich fluid can be supplied from the storage vessel 16 by means of thepump 15 to the well 10. The well 16 is filled to above the top of theformation 12 to be treated with a fracturing fluid 17, while theremaining volume of the well is filled with a driving fluid 18.

FIGURE 1A is a graphical representation of the value of the hydrostaticpressure in the well 19 and the breakdown pressure in the formation 12as a function of the depth of the well. To this end the pressure isplotted on the X-axis, and the height of the column of fracturing fluidrelative to the bottom of the well along the Y-axis. In FIGURE 1, thebottom of the column of fracturing fluid coincides with the bottom ofthe well and the bottom of the formation 12.

As the formation 12 shows a tendency to horizontal fracturing, theformation breakdown pressure is proportional to the sum of the geostaticpressure and the rockbonding strength. Since the latter is usuallynegligible compared to the other stresses, the formation breakdownpressure may be regarded as being almost proportional to the geostaticpressure, and this pressure is in its turn almost proportional to thedepth of the well. The breakdown pressure therefore decreases linearlyfrom the lowest point in the well towards the top. The values of thebreakdown pressure in the formation 12 are shown by the line 19. Thevalue (A) of the breakdown pressure at the top of the formation 12 isdetermined by the weight of the overburden above the formation 12, plusany rockbonding strength at the top of the formation 12. Since thebreakdown pressure is substantially proportional to the geostaticpressure, the slope of the line 19 depends solely on the specificgravity of the rock in the formation 12.

The value of the hydrostatic pressure in the fluid-filled well as afunction of the depth of the well is shown in FIGURE 1A by line 20. Thisline is bent when the fracturing fluid 17 comes into contact with thedriving fluid 18. If no pressure is exerted on the column of fluid bythe pump 15, the extension of the line 20 will intersect the Y-axis at aheight of the column of fluid corresponding to the depth of the well.

Since, in the case under discussion, the formation shows a tendency tohorizontal fracturing, the fracturing fluid has a specific gravitygreater than that of the rock in the formation 12, and hence, preceedingin a downward direction, the increase in the hydrostatic pressure of thefluid in the well per unit of length of the well will be greater thanthe increase in the breakdown pressure in the formation 12 per unit oflength; as a result, the difference between the hydrostatic pressure andthe breakdown pressure is smallest at the level of the bottom of thecolumn of fracturing fluid 17.

The desired specific gravity for the fracturing fluid, which shouldusually be between 2 and 3, is obtained by adding such weightingmaterials as barytes, pyrites, red lead and lead glance (galenite). Thewhole fluid column in the well may, if necessary, consist of fracturingfluid, but it is preferable to endeavor to use a minimum quantity ofweighting materials, thus only part of the well is filled withfracturing fluid. The remainder of the well is filled with a drivingfluid having a lower specific gravity than the fracturing fluid, asreflected in the course of the upper part of line 20 in FIGURE 1A.

After the required quantities of fluid have been introduced into theWell, the pressure of the fracturing fluid is increased. This ispreferably done by introducing fluid under pressure from the storagevessel 16 by means of the pump 15 through the line 14 to the top of thecasing 13 in the Well It). This fluid should preferably have the samecomposition as the driving fluid. A sufficiently high hydrostaticpressure at the bottom of the column of fracturing fluid can be achievedfilling the well with the heavy fracturing fluid instead of drivingfluid. The resultant increase in pressure is propagated without lossthroughout the well until at a certain moment the hydrostatic pressureof the fluid at the lowest part of the well is equal to the breakdownpressure of the formation in that part. This condition is showngnaphically by the line 20'. In the higher parts of the well 10 thehydrostatic pressure is less than the breakdown pressure of theformation in these parts and thus no fractures can occur in these parts.

If the pressure in the well increases still further by the introductionof fluid at the top of the casing 13 corresponding to line 20", thisprocdues a fracture 21. As can be seen from FIGURE 1A, the hydrostaticpressure in parts of the well above the lowest region of the formationremains less than the breakdown pressure in the lowest Zone, thus nofracturing of the formation will occur in these parts.

Since fracturing fluid disappears into the formation after the fracture21 has been produced, the hydrostatic pressure at the bottom of thefluid column in the well should be kept constant by introducing fluid tothe top of the well. The pressure exerted by the fracturing fluidpresent in the resultant fracture will thus remain higher than theformation breakdown pressure thereby extending the fracture 21 into theformation.

It can be seen from the graphical representation in FIGURE 113 that thequantity of fracturing fluid in the well 10 has to be suflicient toprevent the interface between the fracturing fiuid and the driving fluidfrom falling below the bottom of the casing 13. If the interface fallsbelow the casing 13, there is a chance that the hydrostatic pressure ofthe fluid at the top of the formation 12 may exceed the formationbreakdown pressure, thus producing a fracture in the upper portion ofthe formation.

FIGURE 2 shows a second embodiment of the method according to theinvention which permits the fracture to be produced at a place higherthan the bottom of the formation 12. For this purpose a packer element22. completely shutting off the passageway of the well, is setimmediately below the location where the fracture is to be produced.Otherwise, the method is identical to that described in connection withFIGURE 1. In this case, the hydrostatic pressure of the fluid isadjusted until it exceeds the breakdown pressure of formation 12adjacent packer 22.

It is to be noted that if the formation which is to be fractured tendsto form vertical fractures, the formation breakdown pressure is between60 and 70% of the geostatic pressure (plus any rock-bonding strength).In the case of a rock with a specific gravity equal to the specificgravity of the formation 12, the slope of the line 19 which shows thevalue of the breakdown pressure as a function of the height of the wellwill therefore be greater than given in the drawing. By using afracturing fluid having a specific gravity greater than 60 to 70 percentof the specific gravity of the formation rock, and applying extrapressure to the top of the column, equilibrium between the hydrostaticpressure and the formation breakdown pressure is first reached at thebottom of the column of fracturing fluid. When the extra pressure isincreased still further the hydrostatic pressure exceeds the formationbreakdown pressure here and begins to fracture the formation at thispoint along one or more vertical planes. The top of the verticalfractures is determined by the region in the well in which thehydrostatic pressure and the formation breakdown pressure are inequilibrium. The location of this region can be accurately controlled bycontrolling the pressure applied to the column of fluid by the pump 15.

From the above description it is seen that a novel method has beenprovided for controlling the location of fractures created in aformation traversed by a well. The control is achieved by adjusting thespecific gravity of the fracturing fluid adjacent the desired locationof the fracture to develop a pressure gradient that exceeds theformation breakdown pressure gradient. The fracture is created byincreasing the hydrostatic pressure of the fracturing fluid until itexceeds the formation breakdown pressure. Fractures in other formationstraversed by the Well are prevented by controlling the specific gravityof the fluid to maintain the hydrostatic pressure in all cases less thanthe breakdown pressure of the formations.

While but a single embodiment of this invention has been described, manymodifications and improvements are possible within its broad spirit andscope.

I claim as my invention:

A method of fracturing a formation traversed by a well by means of afracturing fluid under pressure comprising:

partially filling the well with a column of fracturing fluid, the bottomof said column adjoining the area at which the bottom of the fracture isdesired and the top of said column of fracturing fluid beingnonconstrained, the specific gravity of said fracturing fluid beingadjusted until the pressure gradient of the column of fracturing fluidexceeds the pressure gradi- References Cited in the file of this patentent of the formation to be fractured; filling the remainder of the wellwith a driving fluid, the UNITFD STATES PATENTS specific gravity of saiddriving fluid being adjusted R6- 23,73 Farris Nov. 10, 1953 until thepressure gradient of the column of driving 5 2,596,845 Clark May 13,1952 fluid is less than the pressure gradient of all forma- 2,838,117Clark et a1 June 10, 1958 traversed by the W611; 2,851,109 Spearow Sept.9, 1958 increasing the pressure on the drlvlng fluid at the sur- 2 906340 Herzon Sept 29 1959 face until the pressure of the fracturing fluidexceeds the breakdown pressure of the formation to be frac- 10 tured.

2,950,247 McGuire et al Aug. 23, 1960

